1. Field of the Invention
The present invention is directed to a web cutting apparatus, and more particularly to an apparatus in which a continuously travelling web is cut into a desired length.
2. Description of the Prior Art
The term "web" referred to in the present invention is a relatively long and flexible belt like substance generally having a thickness of 5 microns to 5 mm and a width of 10 cm to 3 m and comprises, for example, a plastic film of polyvinyl chloride, polycarbonate, acrylonitrile-styrene copolymers, ABS resins, polyesters, polyester resins containing glass fibers, cellulose derivatives or the like, and further, a sheet of paper, synthetic paper or the like, or a thin metal plate such as aluminum, copper or the like.
In general, the above mentioned web is, after once winding up into a roll shape during its manufacturing process, cut into a desired length depending upon its use while rewinding it during a machining step. In order to effectively carry out the cutting, two types of cutting apparatuses have been widely employed. One is a rotary cutter comprising a pair of rotary blades which are placed face-to-face one above and one below the web. The rotary blades are driven with the same circumferential speed as the longitudinal speed of the web and cuts the web into a desired length by engaging respective edges of the rotary blades. The other is called a self running cutter and comprises a freely vertically movable blade above the web and a receiving blade below the web which cuts the web by moving the movable blade downward to the receiving blade while simultaneously moving the movable blade and recieving blade parallel to the web. Because both make it their main purpose to cut the web into a desired length without stopping the running of the web, it has been expected that cutting in accordance with either apparatus would be extremely difficult.
However, in the former, it takes considerable time to determine the engagement of respective edges of the rotary blades for cutting of the web and for adjustment thereof since the rotary blades are held with supporting means which are respectively independent and freely rotatable. In addition, since it is necessary that the circumferential speeds of respective blade edges be identical with the longitudinal speed of the web, mechanical errors found in the freely rotatable supporting means as well as respective driving systems of the web, e.g., slip, backlash, etc., have been obstacles to raising accuracy in cutting length of the web. On the other hand, while the latter greatly improves the disadvantages in determining engagement of respective blade edges and in making speeds uniform and controlling them in respective driving systems with resulting greatly improved accuracies in cutting lengths of the web, it is still not sufficiently accurate for applications requiring much higher accuracy in cutting length, e.g., requiring accuracies on the order of 500 microns or less.
After investigations and analyses with respect to the major causes thereof, the present inventors have confirmed that torque variations having a relatively large level difference are generated whenever the driving system in which the movable blade and receiving blade are simultaneously moved in the direction parallel to the web and the driving system in which the movable blade is moved downward to the receiving blade is operated with a more or less time lag. As a result, uneven rotation is caused in the driving sources mentioned above so that a warp tends to occur at the engagement point between the movable blade and receiving blade, that is, at the cutting point of the web, and this tendency is increased if the running speed of the web increases. In addition, it is generally the case that a common driving source is used to drive the cutting mechanism and the web in order to minimize production costs. Therefore, it was difficult to change the cutting length of the web once it was set, and thus the apparatus was unsuitable for cutting a web in a wide variety of cutting sizes.